Iron(III) oxide-hydroxideorferric oxyhydroxide[2] is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH).
The compound is often encountered as one of its hydrates, FeO(OH)·nH 2O [rust]. The monohydrate FeO(OH)·H 2O is often referred to as iron(III) hydroxideFe(OH) 3,[3]hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.[3]
Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral bernalite, Fe(OH)3·nH2O (n = 0.0–0.25).[4][5] Iron oxyhydroxides, FeOOH, are much more common and occur naturally as structurally different minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.
Goethite, α-FeO(OH), has been used as an ochre pigment since prehistoric times.
Akaganeite is the β polymorph,[6] formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some chloride ions to stabilize its structure, so that its more accurate formula is FeO 0.833(OH) 1.167Cl 0.167orFe 6O 5(OH) 7Cl.[7]
Lepidocrocite, the γ polymorph, is commonly encountered as rust on the inside of steel water pipes and tanks.
Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions.
Siderogel is a naturally occurring colloidal form of iron(III) oxide-hydroxide.
Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxyhydroxide and the most important mineral carriers of iron in soils.
The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.
The crystal structure of β-FeOOH (akaganeite) is that of hollanditeorBaMn 8O 16. The unit cell is tetragonal with a = 1.048 and c = 0.3023 nm, and contains eight formula units of FeOOH. Its dimensions are about 500 × 50 × 50 nm. Twinning often produces particles with the shape of hexagonal stars. [2]
Limonite, a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major iron ores, having been used since at least 2500 BC.[8][9]
Iron(III) oxyhydroxide precipitates from solutions of iron(III) salts at pH between 6.5 and 8.[12]
Thus the oxyhydroxide can be obtained in the lab by reacting an iron(III) salt, such as ferric chlorideorferric nitrate, with sodium hydroxide:[13]
FeCl 3 + 3 NaOH → Fe(OH) 3 + 3 NaCl
Fe(NO 3) 3 + 3 NaOH → Fe(OH) 3 + 3 NaNO 3
In fact, when dissolved in water, pure FeCl 3 will hydrolyze to some extent, yielding the oxyhydroxide and making the solution acidic:[12]
FeCl 3 + 2 H 2O ↔ FeOOH + 3 HCl
Therefore, the compound can also be obtained by the decomposition of acidic solutions of iron(III) chloride held near the boiling point for days or weeks:[14]
FeCl 3 + 2 H 2O → FeOOH(s) + 3 HCl(g)
(The same process applied to iron(III) nitrateFe(NO 3) 3 or perchlorate Fe(ClO 4) 3 solutions yields instead particles of α-Fe 2O 3.[14])
Another similar route is the decomposition of iron(III) nitrate dissolved in stearic acid at about 120 °C.[15]
The oxyhydroxide prepared from ferric chloride is usually the β polymorph (akaganeite), often in the form of thin needles.[14][16]
The oxyhydroxide can also be produced by a solid-state transformation from iron(II) chloride tetrahydrate FeCl 2·4H 2O.[6]
The compound also readily forms when iron(II) hydroxide is exposed to air:
4Fe(OH) 2 + O 2 → 4 FeOOH + 2 H 2O
The iron(II) hydroxide can also be oxidized by hydrogen peroxide in the presence of an acid:
^Safoora Rahimi, Rozita M. Moattari, Laleh Rajabi, Ali Ashraf Derakhshan, and Mohammad Keyhani (2015): "Iron oxide/hydroxide (α,γ-FeOOH) nanoparticles as high potential adsorbents for lead removal from polluted aquatic media". Journal of Industrial and Engineering Chemistry, volume 23, pages 33-43. doi:10.1016/j.jiec.2014.07.039
^ abTim Grundl and Jim Delwiche (1993): "Kinetics of ferric oxyhydroxide precipitation". Journal of Contaminant Hydrology, volume 14, issue 1, pages 71-87. doi:10.1016/0169-7722(93)90042-Q
^K. H. Gayer and Leo Woontner (1956): "The Solubility of Ferrous Hydroxide and Ferric Hydroxide in Acidic and Basic Media at 25°". Journal of Physical Chemistry, volume 60, issue 11, pages 1569–1571. doi:10.1021/j150545a021
^ abcEgon Matijević and Paul Scheiner (1978): "Ferric hydrous oxide sols: III. Preparation of uniform particles by hydrolysis of Fe(III)-chloride, -nitrate, and -perchlorate solutions". Journal of Colloid and Interface Science, volume 63, issue 3, pages 509-524. doi:10.1016/S0021-9797(78)80011-3
^Dan Li, Xiaohui Wang, Gang Xiong, Lude Lu, Xujie Yang and Xin Wang (1997): "A novel technique to prepare ultrafine Fe 2O 3 via hydrated iron(III) nitrate". Journal of Materials Science Letters volume 16, pages 493–495 doi:10.1023/A:1018528713566
^Donald O.
Whittemore and Donald Langmuir (1974): "Ferric Oxyhydroxide Microparticles in Water". Environmental Health Perspective, volume 9, pages 173-176. doi:10.1289/ehp.749173
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